Feline mammary carcinomas are, like human breast cancers, spontaneous, locally infiltrative and metastasizing tumors. Therefore, this tumor disease in the cat can serve as a pathogenetic and experimental-therapeutic model for the human counterpart. In the cat, as in the woman, little is so far known with certainty about the hormonal background of mammary tumors. In order to elucidate the role of endogenous and exogenous hormonal factors, a case-control study was conducted. Data on age, history of castration, parity and progestogen administration were compared in cats with malignant or benign mammary tumors on one hand, and in a control group on the other. The statistical relative risks and their significance were assessed using conditional logistic regression analysis. In our study there was a tendency for mammary carcinomas to be found in cats that were older than those bearing benign mammary tumors. Ovariectomy was found to protect against mammary carcinomas but not against benign mammary tumors. No association between parity and mammary tumor risk was found. Regular administration of progestogens was associated with an increased risk of both mammary carcinoma and benign mammary tumors. However, this was not true of irregular progestogen administration and, in general, the administration of progestogens was not associated with an earlier appearance of mammary tumors.